By conventional production of graphite bodies it is used gas calcined coke. Gas calcined petrol coke contains varying amount of sulphur, typical 0.4-4%. Gas calcined coke particles with a selected particle size distribution are mixed with tar/pitch binder, formed whereafter the bodies are baked at 850-950° C. Thereafter the baked bodies are graphitized in conventional graphitizing furnaces at a temperature in the range of 2300-3300° C. During the graphitizing process sulphur will be released from the coke at a temperature in the range of 1400-2000° C. This is known as “sulphur puffing” and will result in an irreversible thermal expansion of the carbon body which in turn will result in increased porosity, lower density and lower electric and thermal conductivity in the graphite bodies.
It is known to counteract the effect of sulphur puffing during production of graphite bodies by addition of sulphide forming compounds such as Fe2O3, CaN2 and Na2CO3. Other salts or oxides of sulphide forming metals such as manganese, aluminium, zinc and copper have also been used. The addition of such compounds can however have other negative influence on the properties of the produced graphite bodies.
Also nitrogen content in coke, particularly pitch coke, can give rise to puffing during heat treatment in the range of 1400-2100° C. Addition of Cr2O3, CoO, NiO and boric acid have shown to effectively reduce the effect of nitrogen puffing.
In order to reduce the porosity and increase the density the produced graphite bodies made from coke, particularly coke with high sulphur content and possibly nitrogen content, are impregnated with tar pitch under vacuum at temperatures in the range of 200-300° C. and then baked once more at a temperature of at least 750° C. If a 100% graphitizing is needed a second graphitizing step at a temperature in the range of 2300-3300° C. has to be done. If even higher density is needed it may be necessary to repeat the steps of impregnating, baking and graphitizing.
The steps of impregnating under vacuum baking and possibly renewed graphitizing are very costly.
By the present invention one have arrived at a method for production of graphite bodies where the steps of adding inhibitors to reduce the effect of sulphur and nitrogen puffing, impregnating with tar pitch and baking after the first graphitization step and a second graphitization step can be avoided and whereby graphite bodies can be obtained having as good or better properties than conventionally produced graphite bodies that are impregnated with tar pitch and subjected to baking and a second graphitization step. By the present invention it is particularly obtained graphite bodies having a high density, low open porosity and air permeability compared to conventionally graphite bodies.